Gas turbine engines operate at high rotational speeds and high temperatures for increased performance and efficiency. The demand for greater efficiency requires gas turbine engines to operate at gas temperatures in excess of about 2750.degree. F. (1510.degree. C.). Direct exposure to this high temperature gas, however, detrimentally affects turbine airfoils by causing component distortion and even melting in extreme cases.
Airfoil cooling techniques have been developed to keep the metal temperature of the airfoils within design limits while operating in a high temperature environment. Convection and/or film cooling are techniques used in the design of air-cooled turbine airfoils. Improved convection cooling is obtained by using such devices as fins, pins, and narrow multi-pass channels to increase heat transfer surface area inside the airfoil. Convection cooling can also be enhanced by the impingement of high velocity cooling air on the hot airfoil surfaces.
In film cooling, cool air is fed through holes in the airfoil wall to form an insulating layer between the hot gases and the airfoil surface. Since air has a very low thermal conductivity, it is an effective insulation medium. If, however, the cool air exits the holes at too steep an angle to the airfoil surface or at too great a velocity it will blow away from the airfoil surface and be ineffective in cooling the airfoil surface. Effective cooling requires the air to exit the holes at a low angle and low velocity to the airfoil surface.
Turbine airfoil designers rely on several standard cooling hole geometries to obtain cooling. These geometries include round, rectangular, conical, and shaped or "diffuser" holes. Such standard hole geometries, however, cannot sufficiently cool turbine airfoils to survive in an environment where temperatures can reach 3000.degree. F. (1649.degree. C.). More advanced shaped cooling holes, such as curved cooling holes, may provide a significant reduction in airfoil metal temperature and thus greatly improve turbine airfoil cooling. Accordingly, there is a need for complex-shaped cooling holes, such as curved cooling holes, to increase turbine airfoil film cooling effectiveness.